1. Field of the Invention
The invention relates generally to optical transmission apparatus and methods and more particularly, to optical modulators and optical modulation methods.
2. Discussion of the Related Art
This section introduces aspects that may help facilitate a better understanding of the inventions. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is prior art or what is not prior art.
There is a strong interest in optical communication systems that transmit data at high rates. While high transmission rate can be achieved by multiplexing a data stream onto different optical channels in parallel, this simple technique tends to provide a low spectral efficiency. The spectral efficiency may be increased by relying on larger symbol constellations to modulate the data stream onto the optical carrier, e.g., a quadrature phase shift keyed (QPSK) constellation. Some larger symbol constellations can be used by optical modulators in which optical phase shifter(s) are constructed to provide a range of phase shifts. Nevertheless, some such optical phase shifters are complicated to implement due to nonlinearities and/or weak electro-optical responses.